HIV-1 is the etiological agent of acquired human immune deficiency syndrome (AIDS) and related disorders. HIV-1 is an RNA virus of the Retroviridae family and exhibits the same 5'LTR-gag-pol-env-LTR3' organization as all retroviruses. In addition, it comprises a handful of genes with regulatory or unknown function, in particular the tat and rev genes. The env gene encodes the viral envelope glycoprotein that is first translated as a 160-kilodalton (kDa) precursor (gp160), which is subsequently cleaved by a cellular protease to yield the external 120-kDa envelope glycoprotein (gp120) and the transmembrane 41-kDa envelope glycoprotein (gp41). Gp120 and gp41 remain associated and are displayed on the viral particles as well as on the surface of HIV-infected cells. Gp120 is directly responsible for binding to the CD4 receptor present on the surface of helper T-lymphocytes, macrophages and other target cells. After gp120 binds to CD4, gp41 mediates the fusion event responsible for virus entry.
Infection begins as gp120 on the viral particle binds tightly to the CD4 receptor on the surface of T4 lymphocytes or other target cells. The virus then merges with the target cell and reverse transcribes its RNA genome into double-stranded DNA. The viral DNA becomes incorporated into the genetic material in the cell's nucleus and directs the production of new viral RNA and viral proteins, which combine to form new virus particles. These particles bud from the target cell membrane and infect other cells.
Destruction of T4 lymphocytes, which are critical to immune defense, is the major cause of the progressive immune dysfunction that is the hallmark of HIV infection. The loss of target cells seriously impairs the body's ability to fight most invaders, but it has a particularly severe impact on the defenses against viruses, fungi, parasites and certain bacteria, including mycobacteria.
HIV-1 is known to kill the cells it infects by replicating, budding from them and damaging the cell membrane. HIV-1 might also kill target cells indirectly, by means of the viral protein, gp120, that is displayed on an infected cell's surface. The CD4 receptor on T cells has a strong affinity for gp120, and healthy T4 cells and cells expressing CD4 receptor can bind to gp120 and fuse with infected cells. In addition to the CD4-gp120 interaction, other receptors may also play a role in the fusion process. The end result, called a syncytium, cannot survive, and all the once healthy cells it contains are destroyed along with the infected cell. HIV-1 can also elicit normal cellular immune defenses against infected cells. With or without the help of antibodies, cytotoxic defensive cells can destroy an infected cell that displays viral proteins on its surface. Finally, free gp120 may circulate in the blood of individuals infected with HIV-1. The free protein may bind to the CD4 receptor of uninfected cells, making them appear to be infected and evoking an immune response.
Infection with HIV-1 is almost always fatal, and at present there are no cures for HIV-1 infection. Effective vaccines for prevention of HIV-1 infection are not yet available. Because of the danger of reversion or infection, conventional live attenuated virus or killed whole virus cannot be used as vaccines. Also most subunit vaccine approaches have not been successful at preventing HIV infection to date. In addition, treatments for HIV-1 infection, while prolonging the life of infected persons to some extent, have serious side effects. There is thus a great need for effective treatments and vaccines to combat this lethal infection.
Vaccination is an effective form of disease prevention and has proven successful against several types of viral infection. Determining ways to present HIV-1 antigens to the human immune system in order to evoke protective humoral and cellular immunity, is a difficult task. At the present time most attempts to generate an effective HIV vaccine have been unsuccessful. In AIDS patients, free virus is present in low levels only. Transmission of HIV-1 is enhanced by cell-to-cell interaction via fusion and syncytia formation. Hence, antibodies generated against free virus or viral subunits are generally ineffective in eliminating virus-infected cells.
Vaccines exploit the body's ability to "remember" an antigen. After first encounters with a given antigen the immune system generates cells that retain an immunological memory of the antigen for an individual's lifetime. Consequently, subsequent exposure to the antigen results in elimination or inactivation of the pathogen. The immune system deals with pathogens in two ways: by humoral and by cell-mediated responses. In the humoral response lymphocytes generate specific antibodies that bind to the antigen thus inactivating the pathogen. The cell-mediated response involves cytotoxic lymphocytes that specifically attack and destroy infected cells.
Vaccine development with HIV-1 virus presents problems because the virus infects some of the same cells the vaccine needs to activate in the immune system (i.e., T4 lymphocytes). Therefore it would be advantageous for the vaccine to inactivate the HIV before impairment of the immune system occurs. A particularly suitable type of HIV vaccine would generate an anti-HIV immune response which will recognize innumerable HIV variants and will extend its activity to HIV-positive individuals who are at the beginning of their infection.
It is accordingly an object of the invention to provide vaccines for use in the prevention and treatment of HIV-1 infection. It is also an object of the invention to provide methods for screening compounds that inhibit the deleterious syncytia formation for use as treatments for HIV-1 infection.
Sodroski, J. et al. (1986) Nature 322: 470-474 discloses T4+ Jurkat-tat-III cells (a T4+ Burkitt's lymphoma cell line that expresses the HIV-1 tat gene product) transfected with a plasmid designed to express both the rev and env gene products. These cells transiently expressed HIV-1 gp160, and were used in a syncytia formation assay in studies on the basis of the specific cytotoxicity of the AIDS virus. Although the cells expressed HIV-1 gp160, transient expression of the envelope protein is inconvenient and time-consuming for use in syncytium formation assays, since new cells would have to be transfected at frequent intervals.
Palker, T. J. et al (1987) Proc. Natl. Acad. Sci. USA 84: 2479-2483 also discloses a syncytia formation assay. The assay was used for the study of antibodies specific for HIV-1 proteins.